Abstract

Angiopoietin-1 is a Tie-2 receptor agonist that stabilizes vascular endothelium, promoting
endothelial maturation and preventing capillary leak. Angiopoietin-2 is largely a
competitive partial antagonist that is markedly elevated in humans and animal models
of sepsis and other inflammatory states, directly disrupts the endothelial barrier,
and has been correlated with end-organ dysfunction and death in sepsis. In the previous
issue of Critical Care, Alfieri and colleagues used intravital microscopy to study the microvasculature
in a murine model of sepsis. Treatment with a modified angiopoietin-1 molecule led
to reversal of albumin vascular leak and improved blood flow to skeletal muscle, as
well as a decrease in the levels of several inflammatory cytokines. Importantly, the
angiopoietin-1 variant was administered 20 hours after initial lipopolysaccharide
challenge. This study adds to the evidence that the angiopoietin/Tie-2 axis represents
a modifiable pathway through which targeted therapy may be able to directly reverse
part of the pathology of sepsis.

Commentary

Despite decades of research and untold investments of time, energy, and money, treatments
for sepsis other than antibiotics and supportive care have remained elusive. Over
this time, our expanding understanding of the biology of sepsis has led to the development
of targeted treatments, including immunomodulatory and anti-coagulant approaches,
yet so far all of these approaches have been ultimately unsuccessful.

With this historical backdrop, much recent translational research in sepsis has focused
on the importance of vascular endothelial permeability, a pathophysiologic hallmark
of the syndrome and the focus of the work of Alfieri and colleagues in the previous
issue of Critical Care [1]. The angiopoietin (Ang)-1/2 axis is a key regulator of endothelial permeability,
operating via the Tie-2 receptor on vascular endothelial cells. Ang-1, the primary
Tie-2 agonist, decreases capillary leak and inhibits leukocyte-endothelial interaction,
among other effects [2-4]. In contrast, Ang-2, produced primarily in endothelial cells, functions as a context-dependent
Tie-2 antagonist [5,6]. Elegant experimental and translational studies have clearly demonstrated that Ang-2
plays a critical role in the organ injury of sepsis by mediating increased endothelial
permeability [7,8]. Likewise, clinical studies have identified a strong association between lower Ang-1
and/or higher Ang-2 levels and poor clinical outcomes in sepsis, including acute lung
injury, pulmonary leak index, multi-organ dysfunction, and mortality [6,9-11]. Thus, manipulation of the Ang-1/2 axis has become an appealing therapeutic target.

One challenge of therapeutic intervention on the Ang-1/2 axis is that recombinant
human Ang-1 has poor solubility and a short half-life [12]. Thus, alternative approaches to effective delivery of Ang-1 have been investigated
in experimental models. Cartilage oligomeric matrix protein-angiopoietin-1 (COMP.Ang-1)
is a soluble and stabilized variant that binds more avidly to Tie-2 than native Ang-1
[12]. COMP.Ang-1, adenovirus-delivered Ang-1, recombinant human Ang-1, cell-based therapies,
and a synthetic Tie-2 agonist all reduce vascular leak and end-organ dysfunction in
murine sepsis models [13-19]. Each of these approaches has some appeal; Alfieri and colleagues chose to investigate
the effects of a modified Ang-1 molecule, MAT.Ang-1, formed by fusing the coiled-coil
domain of human matrilin-1 to the fibrinogen-like domain of human Ang-1. MAT.Ang-1
has better solubility than recombinant Ang-1 and yet more similar biologic activity
to native Ang-1 than COMP.Ang-1 [12].

To study the effects of this modified Ang-1 in experimental models of sepsis, Alfieri
and colleagues implanted window chambers into the dorsal skinfolds of mice, so as
to visualize skeletal muscle blood flow with laser Doppler imaging. Lipopolysaccharide
(LPS) was injected intraperitoneally at 0 and 19 hours, followed by intravenous MAT.Ang-1
at 20 hours. MAT.Ang-1 returned LPS-induced leak of albumin to control levels at 23
and 24 hours, without affecting vascular permeability in mice untreated with LPS.
Microvascular perfusion at 24 hours partially improved in LPS-challenged mice treated
with MAT.Ang-1, but did not return to normal. In addition to its effects on vascular
permeability and perfusion, MAT. Ang-1 reduced tumor necrosis factor alpha, interferon
gamma, triggering receptor expressed on myeloid cells-1, granulocyte colony-stimulating
factor, and IL-10 in LPS-unchallenged mice, a novel finding given that other studies
have not treated well mice with Tie-2 agonists. LPS-induced elevation in IL-1β, IL-10,
and IL-1 receptor antagonist was decreased by MAT.Ang-1, consistent with two other
Tie-2 agonists' effects on inflammatory cytokines [14,20].

What can we learn from this new study, and how has it advanced the field? First, MAT.Ang-1
was given after the administration of LPS, rather than prophylactically. While the
therapeutic interval was short, this treatment model highlights the possibility that
a Tie-2 agonist could be a feasible therapy for the microvascular dysfunction of sepsis.
Second, although MAT.Ang-1 was described previously, this study represents its first
therapeutic trial in this setting. Third, the in vivo imaging of skeletal muscle provides an interesting window into microvascular function
and a novel demonstration of the potential therapeutic effects of manipulation of
the Ang1/2 axis.

At the same time, many questions about the effects of this modified Ang-1 compound
remain unanswered. In the current paper, only effects on skeletal muscle vasculature
and expression of cytokines and angiogenic factors were assessed. What might lung
intravital microscopy, which has become a valuable scientific tool in models of acute
lung injury, reveal about the effects of MAT.Ang-1 on the pulmonary vascular bed and
leukocyte tracking in the lung? More broadly, what are the effects of MAT.Ang-1 on
organ injury and mortality in experimental models of sepsis? How does MAT.Ang-1 affect
the immune response, including bacterial counts, in live bacterial models? Perhaps
most importantly, how do different approaches to manipulating the Ang-1/2 axis compare
in the same experimental sepsis models?

While many questions remain, this work by Alfieri and colleagues adds to the growing
weight of literature highlighting the potential value of targeting the vascular endothelium
in sepsis and will certainly pique the interest of the many researchers who are continuing
the decades-long quest for the holy grail of a targeted sepsis therapy.

Abbreviations

Competing interests

CSC receives research funding from the National Institutes of Health, the Flight Attendant
Medical Research Institute, and Glaxo Smith Kline and has served on medical advisory
boards for Ikaria, Cerus Corporation, and Glaxo Smith Kline.